Segments of the personal computer (PC) industry have dramatically changed during the last decade. The future is even more challenging. A dramatic increase in the processor speeds of PCs has required an overwhelming increase in current and associated dynamics (very high slew rate). This already challenging technical requirement is further complicated by a need for voltage reduction, potentially to sub-volt levels.
In the past, there was virtually no challenge in powering computers. A multiple output, very slow power supply called a "Silver Box (SB)" was adapted to meet the requirements of every power demand. However, as silicon development progressed, multiple voltages of less than 3.3V were required. Voltage Regulator Modules (VRMs) on the processor Mother Board (MB) were a logical solution to that problem. Today, the number of VRMs required on the Mother Board is increasing. In addition to the VRMS, a large number of de-coupling capacitors are required in proximity of the processor to meet the requirements of very high slew rate of the current. This has resulted in a rapid increase in the cost, as well as a large reduction in overall efficiency, of the power delivery system.
A number of options for improving this situation have been explored. For example, Advanced Voltage Regulator Module (AVRM) offers the capability to supply high di/dt and high current, however, at increased cost, and with low efficiency and moderately high capacity of the de-coupling capacitors. Replacing low voltage DC distribution with higher DC voltage, such as 48V, is more promising but has a drawback of higher cost. Recently a novel High Frequency Alternating Current (HFAC) power delivery architecture has been proposed for powering the future generation PCs in a reference entitled, "PC Platform Power Distribution System: Past Application, Today's Challenge and Future Direction" published in the conference proceedings of International Telecommunications Energy Conference, Copenhagen, Denmark, June 1999 by J. Drobnik, L. Huang, P. Jain and R. Steigerwald. In the HFAC architecture, the system power supply (silver box) generates high frequency and high voltage. The HFAC is then fed to an individual ac-dc converter (ACVRM) and converted into DC of specific parameters at the point of use. The HFAC brings the following advantages:
Low cost; PA1 High efficiency; PA1 Better form factor; PA1 Better reliability; and PA1 Active energy steering.
HFAC is conceptually the simplest architecture proposed to date, which deals with all of the power delivery issues defined above. This includes elimination of duplicated power conversions, and active energy steering without additional components.
The key to successful implementation of an HFAC power delivery system resides in the stage that converts high frequency AC to dc. A number of conventional approaches for this conversion are discussed below.
FIGS. 1(a)-(d) show various known topologies for converting high frequency AC to dc.
FIG. 1(a) shows a converter topology that includes a series resonant circuit, a transformer, diode rectifiers, an output capacitor, a switch operating in a linear range, and a second output capacitor. This circuit is a straightforward AC to DC converter but suffers from excessive conduction losses, due to the use of the rectifying diodes and the linear output switch. The conduction losses are greatest at the low output voltages and high output currents.
A buck converter, shown in FIG. 1(b), can replace the linearly controlled output switch in FIG. 1(a). This arrangement reduces the conduction losses to a certain extent, but increases the switching losses. It also increases control complexity and provides a sluggish transient response due to the presence of an output inductor. A synchronous buck converter operating in a discontinuous mode, shown in FIG. 1(c), can also replace the linearly controlled output switch shown in FIG. 1(a).
The arrangements shown in FIGS. 1(a)-(c) improve the transient response but increase conduction losses significantly.
FIG. 1(d) shows a circuit diagram for converting AC to regulated DC that suffers from high conduction losses due to the diode rectifying stage in the output. Besides, this circuit is unsuitable for high frequency operation because it uses thyristorized switches at the input to control the output voltage.
It is clear from the above discussion that the conventional approaches to converting high frequency AC to regulated DC have low conversion efficiency due to high conduction and/or switching losses, sluggish transient response and complex control circuitry.
There therefore exists a need for a simple power and control circuitry with high conversion efficiency and fast transient response.